Incorporate independence checking into

[seddonym]

ImportGraph.find_illegal_dependencies_for_layers should support layers at the same level, along these lines:

dependencies = graph.find_illegal_dependencies_for_layers(
    layers=(
        "blue",
        ("red", "green", "orange"),
        "yellow",
    ),
)

This will treat dependencies between sibling layers as illegal. For example, a route from red to green would be an illegal dependency.